In many applications an amplifier is used to increase the power of an input signal. However, during amplification, errors are introduced into the signal. For example, ideally an amplifier will be linear but amplifiers are generally only linear within certain limits resulting in clipping and distortion. For this reason, various mechanisms have been developed to compensate for the non-linearity of an amplifier to increase the limits within which a linear response can be obtained. Two known methods of compensating for the introduction of errors are feedforward linearisation and predistortion.
An example of a known feedforward system, 10, is illustrated in FIG. 1. In this feedforward system 10, the input signal 12 is split into a main signal 14 and a reference signal 16. The main signal passes to a main amplifier 22 where it is amplified. The signal output of the amplifier includes, in addition to the amplified main signal 14, any distortion introduced by the amplifier 22. Once the main signal 14 has been amplified it is sampled and subtracted from the reference signal (at 28) to produce an error signal 30. The error signal 30 ideally only contains errors introduced by the main amplifier 22 when processing the main signal 14. In order to achieve this the main signal and reference signal must be precisely aligned in time and amplitude when they are combined. To achieve this alignment a delay circuit 18 is present in the reference signal path.
The error signal 30 can then be amplified using an error amplifier 32 and subtracted from the output of the main amplifier 22 to remove the errors present in the main amplifier's output 24. Again this requires precise synchronising of the signals to ensure that the error is accurately removed from the signal.
As will be understood by the skilled person one of the main disadvantages of feedforward correction arises from the relatively large distortion from the power amplifier. As the introduced distortion is large a large error amplifier is required to handle the error signal. Additionally, the feedforward loop's gain, phase and delay must be very accurately matched in order to achieve the high levels of cancellation required to reduce the distortion to comply with typical spectral emissions masks. To realise this accuracy often requires the use of additional hardware (pilot tone receivers etc) which further adds to cost and complexity.
An example of a known predistortion system 40 is illustrated in FIG. 2. In a predistortion system the input signal first passes through a predistorter 42 where it is combined with predistortion coefficients that inversely model the amplifier's gain and phase distortion characteristics. The signal then passes through a DAC 44, an up-converter 46 to a power amplifier 48 where it is amplified. The processing by the predistorter means that any errors introduced by the amplifier have already been compensated for in the signal. Optionally, an up-converter (not shown) may be present between the DAC and power amplifier.
The resultant signal may also be used to train the predistorter using a feedback loop 50, including an ADC 46, so that it more accurately models the amplifier's characteristics. This results in greater accuracy in the cancellation of any errors introduced by the amplifier 48. In this way the linearity of the system can be further improved. Optionally, the feedback loop 50 may also include a down-converter (not shown).
In classical predistortion systems the bandwidth of the components in the system are generally required to be at least five times the desired bandwidth of the output signal. Additionally, the components generally have high dynamic range to ensure spectral masks for the output signals are met. To meet these requirements the DSP and DACs used within the power amplifier have a relatively high power consumption and are costly.
Communication systems using multi-carrier signals are particularly challenging for a predistortion system as they have increased dynamic range and bandwidth. Current predistortion systems cannot meet the stringent spectral emissions masks for multi-carrier GSM. It is therefore desirable to further minimise the distortion introduced during the amplification process and improve the efficiency of the linearisation process further.